The present invention relates to a processing unit for use in an electrophotographic type image recording apparatus such as a copying machine, a printer and a facsimile, and to the image recording apparatus incorporating the processing unit.
According to a conventional electrophotographic type image recording apparatus, electrostatic latent image is formed on a charged photosensitive member such as a photosensitive drum upon exposure, and the latent image becomes visible upon developing operation, and the visible image is electrostatically transferred onto an image recording medium by an image transferring process. Such sequential process is generally referred to as "Carlson process".
Further, widely available is a replaceable processing unit in which various components such as a photosensitive drum and expendable such as developer materials for image formation are assembled and accommodated. If the components' life span is expired and the expendables are used up, the unit is replaced with a new processing unit. Examples of such processing unit is disclosed in a Japanese Patent Publication Nos. 58-54392 and 63-9237.
A conventional laser printer incorporating therein the processing unit is shown in FIGS. 1 and 2. A laser printer 10 has a lower box frame 12 and an upper box frame 11 which is pivotally movable about an axis of a support shaft 13 relative to the lower box frame 12. In the upper box frame 11, a processing unit 30 having a photosensitive drum 31 for image formation is detachably assembled. Further, in the upper box frame 11, there are provided a scanner unit 21 and a discharge tray 18. The scanner unit 21 is adapted for scanning laser beam modulated by an image recording signal onto the photosensitive drum 31. The discharge tray 18 is adapted for receiving sheets on which visible images are formed. On the other hand, the lower box frame 12 houses therein a sheet cassette 14 in which fresh sheets are stored, a sheet feed roller 15, a sheet guide 16 which defines a sheet path, a charger 17 for electrostatically transferring a visible image on the photosensitive drum 31 onto the sheet, and an image fixing unit 20 for fixing the transferred image on the sheet.
The processing unit 31 has an outer frame formed with an opening 33 through which the scanning laser beam is passed for forming the electrostatic latent image on the photosensitive drum 31. The processing unit 30 houses therein the photosensitive drum 31 as a photosensitive means, a primary charger 32 for uniformly charging a surface of the photosensitive drum 31, a developing means 34, and a cleaning means 37. The developing means 34 is adapted for developing the electrostatic latent image on the photosensitive drum 31 with toner to form a visible toner image. The cleaning means 37 is adapted for scraping the residual toner from a surface of the photosensitive drum, the toner having not been transferred onto a sheet S at the time of transferring process by means of the charger 17 and for collecting the residual toner. The processing unit 30 is detachably attached to the upper box frame 11 through a unit support (not shown).
If sheet jamming occurs during printing operation of the laser printer 10 or, if the processing unit 30 is to be replaced with a new unit, the upper box frame 11 is pivotally moved upwardly about the axis of the support shaft 13 as shown in FIG. 2, so that the sheet feed path defined by the sheet guide 16 is exposed. Thus, the sheet S can be removed or the processing unit 30 can be exchanged with a new unit.
On the other hand, when the upper box frame 11 is opened, the photosensitive drum 30 may be exposed to an external light to lower charging efficiency. Further, the photosensitive drum is recently formed of an organic photo conductor (OPC) material since the material sufficiently yields productivity and facilitates scrapping. This OPC material has lower surface hardness in comparison with a selenic alloy conventionally used as the material of the photosensitive member. Therefore, the surface of the OPC drum may be damaged at the time of sheet removal work done by an operator. Furthermore, the photosensitive surface of the drum may be contaminated with oils and fats if the operators finger touches the surface. Thus, toner may be adhered to the oil affixed portion at the time of developing operation to degrade final output image.
In order to overcome these drawbacks, a protective cover has been proposed which covers the surface of the photosensitive drum when opening the upper box frame 11. For example, as disclosed in a Japanese Utility Model Application Kokai (OPI) No. 57-123238, a protective cover is movable cooperably with the opening/closing operation of a sheet feed unit.
Further, Japanese Patent Publication No. 1-45635 discloses a protective cover shown in FIGS. 3 and 4. This publication discloses a drum protective cover 60 movable between a drum covering position (FIG. 4) for covering an exposed surface of a photosensitive drum and a retracted position (FIG. 3) away from the exposed surface thereof. The protective cover 60 is a rigid extrusion molded resin product having a curvature substantially coincident with a curvature of the photosensitive drum.
The protective cover 60 is supported rotatably by one end of a first swing arm 61 whose another end is supported rotatably and coaxially with a drum 31 of a processing unit. The protective cover is also supported rotatably by one end of a second swing arm 62 whose another end is fixed to a rotatably shaft 63. The first swing arm 61 is urged in a counterclockwise direction in FIG. 4 by a biasing means. The rotatable shaft 63 is fixed with an operation level 65. Further a projection upstandingly extends from a lower frame.
Since the first swing arm 61 is biased in the direction, the protective cover 60 is urged to be positioned into the drum covering position (FIG. 4). However, if an upper frame is closed, the operation lever 65 is brought into abutment with the projection 66 so as to rotate the second swing arm 62 in a clockwise direction against the biasing force of the biasing means. Therefore, the protective cover 60 is brought into the retracted position to expose the photosensitive drum 31.
In the above described conventional device shown in FIGS. 1 through 4, developer toners accumulated in the developing means 34 (FIG. 1) of the processing unit 30 may undergo consumption in accordance with the numbers of printing. On the other hand, toners which have not been transferred but remained on the drum are collected into the cleaning means 37. Further, chargeability and exposure characteristic of the photosensitive drum 31 may be degraded in accordance with the repeated numbers of the printing process. For example, according to the conventional photosensitive drum formed of OPC and having a diameter of 30 mm, the drum has a service life capable of printing about 3000 to 5000 sheets of A4 size sheets. Therefore, such expendable supplies must be replaced by new supplies at every predetermined numbers of printings.
In order to reduce labors for such replacement and to eliminate any trouble of leakage of wasted toners at the time of the replacement, one unit type processing unit has been widely used in which the processing means are accommodated in a one integral assembly unit. Such integral unit is designed such that the service life of the photosensitive drum, use-up period of the developer toners in the developing means 34 and filling-up period of the wasted developer toners collected by the cleaning means 37 are approximately coincident with one another. With this arrangement, the integral processing unit is replaced by a new integral processing unit when all these means substantially simultaneously reach the service lives. Thus, expedient replacement is achievable.
However, in a recent improvement on OPC material, a photosensitive drum having a diameter of 30 mm can provide prolonged service life capable of performing printing from 10,000 to 20,000 numbers of sheets. Toner amounts to be used for printing 10,000 numbers of sheets may be about 600 grams assuming that printing area ratio is 5% and the toner is made of one component type magnetic toners, even though the toner amount may be dependent on the developing mode and method. Toner transferring efficiency given by the charger may generally be from 75% to 85%. Therefore, about 120 grams of toners are collected in a waste toner container of the cleaning means. Therefore, the volume of the collected toners is about 240 cc assuming that bulk density of the is 0.5 g/cc.
In order to provide the consistency of developing means and the cleaning means with the service life of the photosensitive drum which can have much prolonged service life, the developing means and the cleaning means must have large volume several times as large as the conventional volumes so as to accumulate larger volume of the developer toners therein.
A toner container of the developing means may be easily replaced by a new container if the toners are used up. However, it would be rather difficult to discharge wasted toners in a container of the cleaning means into another container in case of the integral type processing unit, due to the problem of leakage of the wasted toners. Consequently, the integral type processing unit must provide large volume of the wasted toner container of the cleaning means for facilitating replacement work.